The invention pertains to crude oil and additives intended to improve exploitation conditions.
Crude oil can contain large fractions of waxes, the exact nature and quantity of which vary from oilfield to oilfield. At the temperature of the well, the waxes are liquid and dissolved in the crude oil. As the oil rises to the surface, its temperature drops and the waxes crystallise to form a three-dimensional network of needles and flakes. This results in a loss of fluidity and renders production, transport, storage and even treatment of such oil very difficult. Pipeline and unit blockages are frequent.
A number of processes have been proposed to overcome the problem, such as mechanical scraping or heating the walls. Such processes are expensive and they cannot always be carried out.
SHELL has pioneered improving the rheology of crude oil: its French patent FR-A-1 575 984 discloses macromolecular xe2x80x9ccombxe2x80x9d type compounds constructed on a model of a principal hydrocarbon chain onto which are grafted side chains, themselves fairly long hydrocarbons, i.e., at least 14 carbon atoms and at most 30 carbon atoms, to perturb crystallisation of the heavy waxes. That property is well developed in macromolecules with a mass average molecular mass Mn (the definition of which, of course, is: Mn=xcexa3iNiMi/xcexa3iNi, where Mi is the molecular mass of individual species Ni present in the polymer) in the range 1000 to 1000000, preferably in the range 4000 to 100000.
The prior art then suggested the use of additives, usually polymeric additives the role of which was to retard or modify wax crystallisation and thus to improve the flow properties of the oil and prevent agglomeration of the crystals formed on the walls.
A number of studies then attempted to improve the efficiency of these first polymeric additives either by synthesis or by formulation, to adapt them to the different types of crude oils encountered, and to successively overcome the difficulties with synthesis and/or manipulation of the different generations of products, the most efficient examples of which are copolymers of C18-C30 acrylates, preferably mainly C20-C22, with a heterocyclic monomer, in particular vinylpyridine (U.S. Pat. No. 2,839,512 (1958) and FR-B-2 128 589 (1972) from SHELL).
It was shown that the presence of polar moieties endowed the copolymer with a dispersing nature, which prevented deposition of the waxes on the walls. Because of the higher reactivity of long chain acrylates compared with that of polar co-monomers, incorporating the latter is generally very difficult and the dispersing effect, linked to the degree of incorporation of the polar co-monomer, thus usually remained very low.
Despite such successive improvements, prior art additives could not be universally applied to crude oil as each type is a special case with its own problems.
International patent application WO-A-97/34940 proposed particularly high performance wax inhibitors of an alkyl (meth)acrylate copolymer type or alkyl (meth)acrylate and vinyl pyridine (2-vinylpyridine and/or 4-vinylpyridine) copolymer type, but manipulating the vinylpyridines, which are particularly toxic, during their preparation constituted a hurdle to their industrial development despite excellent inhibiting properties.
It has just, unexpectedly, been discovered that alkyl (meth)acrylate copolymer type wax inhibitors perform as well as the wax inhibitors described in WO-A-97/34940 and have the added advantage of not containing any residual reactant in their final composition with associated dangers of high toxicity; these wax inhibitors are copolymers of alkyl acrylates and/or methacrylates and N-vinylpyrrolidone, wherein a portion of the alkyl acrylates and/or methacrylates monomers participating in the polymeric chain can be represented by the formula: 
where R is H or CH3, Ri represents residues of saturated linear aliphatic alcohols Rixe2x80x94OH, wherein the number of carbon atoms is from about 10 to about 50 and originates from an acrylic cut with a particular distribution of alkyl chains, denoted a xe2x80x9cUxe2x80x9d distribution for the purposes of the present patent. The term xe2x80x9cU distributionxe2x80x9d means that the distribution of alkyl chains as a function of the chain length, in this case all with an even number of carbon atoms, wherein the envelope is very regular, wherein the mass average molecular weight Mw is in the range 375 to 700, wherein the number average molecular weight Mn is in the range 375 to 840, and wherein the polydispersity factor Pd=Mw/Mn is in the range 1.0 to 1.2 (Mw is the mass average molecular weight the formula for the calculation of which is Mw=xcexa3iNiMi2/xcexa3iNiMi, where Mi is the molecular mass and Ni the individual species present in the polymer). FIG. 1 shows the distribution of alkyls distributed in accordance with such a xe2x80x9cUxe2x80x9d distribution law with a mean molecular mass of 425 (to produce the corresponding alcohols, see U.S. Pat. No. 4,426,329). The polymeric acrylates or methacrylates obtained by a single polymerisation of monomers with a xe2x80x9cUxe2x80x9d distribution are not particularly distinguished from those obtained from arbitrary monomers, meaning the products normally available to the skilled person and in which no particular distribution for the pendent chain length is sought; in other words wherein the distribution is of any type, in any case not a xe2x80x9cUxe2x80x9d distribution. Surprisingly, and from this the Applicant draws all advantageous consequences, a powerful synergy is developed as regards inhibition of the crystallisation of waxes in crude oil when products of the xe2x80x9cUxe2x80x9d class and of the xe2x80x9cnon Uxe2x80x9d class are distributed in the same (meth)acrylate/N-vinylpyrrolidone copolymer. Like any synergy in mixtures which may be highly variable in composition, the rules are difficult to discern, but the directing principles can be stated and are of great importance to the skilled person: the xe2x80x9cUxe2x80x9d components are centred on average lengths of the pendent chains iu that are longer than those ini of the xe2x80x9cnon Uxe2x80x9d components and the mass in the copolymer of the ensemble of moieties with xe2x80x9cUxe2x80x9d chains is relatively low compared with that of the ensemble of xe2x80x9cnon Uxe2x80x9d moieties. The copolymers of the invention contain 1% to 10% of N-vinylpyrrolidone.
It should be noted that in the copolymers of the invention, the N-vinylpyrrolidone can be at least partially replaced by other vinyllactames, such as vinylbutyrolactame or vinylcaprolactame.
Regarding a structural description, it can be said that the invention is constituted by alkyl (meth)acrylate/N-vinylpyrrolidone copolymers with a mass average molecular weight Mw in the range 5000 to 500000, preferably in the range 40000 to 350000, wherein the acrylate or methacrylate monomer moieties that participate in the polymer chain are moieties representable by the formula: 
where R is H or CH3, Ri are residues of saturated linear aliphatic alcohols Rixe2x80x94OH where i represents the number of carbon atoms of said residues and is in the range 10 to 50 and follow a distribution law that is the superimposition of a xe2x80x9cUxe2x80x9d distribution law, where numbers i are even numbers developed over the high 24-50 portion of the interval, wherein the centred value is iu, and a xe2x80x9cnon-Uxe2x80x9d distribution law where i are even or odd numbers developed over the low portion 10-22 of the interval and wherein the centred value inu is such that inu less than iu, the weight ratio of the ensemble of moieties with formula: 
to Ri distributed in accordance with the xe2x80x9cUxe2x80x9d law to the ensemble of moieties distributed in accordance with the xe2x80x9cnon-Uxe2x80x9d law being 1:99 to 50:50, preferably 5:95 to 50:50.
Formulations for wax inhibitors incorporating these copolymers as essential components overcome the disadvantages cited above and enable a series of additives to be produced with a broad spectrum of use endowed with good solubility in crude oil, which have an effect both on wax crystallisation and on the dispersion of crystals that have already formed. They retard the crystallisation of waxes wherein the distribution is usually ended between C60 and C70, reducing the pour point and viscosity of these oils and facilitating transport, storage and treatment. They are readily incorporated into crude oil of a wide variety of origins.
The copolymers of the invention can be obtained using a simple polymerisation process that is easy to carry out and uses low toxicity reactants.
The polymeric additive of the invention is obtained by polymerising monomers in an aliphatic or aromatic solvent with a boiling point of less than 300xc2x0 C., which is chemically inert towards monomers and in which both the monomers and the copolymer are soluble. Examples of solvents that can be cited are toluene, xylene and aromatic cuts with boiling points centred on about 190xc2x0 C. The polymerisation temperature can vary quite widely as a function of the radical initiator used, for example between 50xc2x0 C. and 150xc2x0 C., preferably between 70xc2x0 C. and 120xc2x0 C. The pressure can be between atmospheric pressure and pressures of 30 bars or less.
The catalysts are generally selected from compounds that generate free radicals that are soluble in the reaction medium, for example organic peroxides such as benzoyl, acetyl or di-tertiobutyl peroxide, or organic peracids or their salts such as tertiobutyl perbenzoate or tertiobutyl peroctoate, or azo compounds such as azo-bis-isobutyronitrile. They can be added as polymerisation is commenced or continuously during polymerisation. In general, 10xe2x88x925 to 10xe2x88x921 moles of free radical generator (or radical initiator) is used, preferably 5xc3x9710xe2x80x944 to 10xe2x88x922 moles per mole of monomer. By weight, a concentration of 0.1% to 1% with respect to the monomers can be used, for example.
The overall concentration of monomers in the solvent can be from 10% to 90% by weight, preferred concentrations being 20% to 70% to control the molecular weight and pumpability of solutions containing the polymeric additive. The degree of polymerisation is measured by gel permeation chromatography (GPC) to produce the mass average Mw and number average Mn molecular masses in polystyrene equivalents and the polydispersity index Pd of the polymer.
In a preferred manner of preparing the copolymers of the invention, the procedure described in the French patent application entitled xe2x80x9cProcess for copolymerising acrylates and/or methacrylates and N-vinylpyrrolidonexe2x80x9d filed on the same day by the same Applicants can be used.
In this preferred procedure, at the end of the copolymerisation step, a small quantity of water and a small supplemental quantity of radical initiator are added to produce complete conversion of the N-vinylpyrrolidone. Further, it has been observed that under these conditions, continuing polymerisation does not result in homopolymerisation of the monomer in the aqueous phase which would result in the presence of insoluble polyvinylpyrrolidone in the finished product, as polyvinylpyrrolidone is insoluble in aromatic solvents.
The quantity of water added is generally 1% to 10% by weight of the reaction mass, preferably about 5% thereof. The supplemental quantity of radical initiator is generally about 5% to 15% of the initial quantity, preferably about 10%.
The reaction is continued until the N-vinylpyrrolidone and acrylic monomers have been completely converted. Water is then eliminated by distillation to result in an organic solution of directly usable copolymer, which generally contains less than 200 ppm of non polymerised N-vinylpyrrolidone.
The mass average molecular mass Mw and number average molecular mass Mn of the copolymers obtained can vary widely depending on the nature of the crude oil to be treated, namely in the range 5000 to 500000 for Mw, preferably in the range 40000 to 350000, for a polydispersity Pd in the range 1.5 to 7.5.
The copolymers of the invention are used in crude oil in amounts that can vary widely depending on the nature, structure and molecular mass of the copolymer to be used, the nature and quantity of the paraffin wax present in the crude oil and the desired performance as regards the reduction in the pour point; these can be in the range 5 to 5000 ppm by weight, preferably 10 to 2000 ppm. They have a favourable influence on the rheology of crude oils, in particular on their viscosity characteristics as a function of temperature and the shear modulus, which in particular controls the pressure necessary to restart a stopped installation (pipeline, well), on their pour point or coagulation temperature, on their starting crystallisation point, on their gravitational flow, on deposits forming in contact with cold walls. All characteristics are important for extraction, transport and storage of oil, and thus several illustrations will be found in laboratory tests intended to appreciate the efficiency of these additives.
The wax deposit inhibiting compositions of the invention are constituted by solutions of these copolymers or additives in concentrations of 2% to 90% by weight, preferably 20% to 70%, in solvents that are soluble in the crude oil to be treated and that can advantageously be solvents used for the polymerisation step selected from aliphatic or aromatic solvents with a boiling point of less than 300xc2x0 C., in particular toluene, xylene or aromatic cuts with boiling points centred, for example, on about 190xc2x0 C.
The invention also provides the skilled person with the possibility of selecting, using simple routine tests, the alcohol cut which will have the greatest similarity with the crude oil to be treated and which will have the highest effectiveness.
The best results are obtained with statistical xe2x80x9cUxe2x80x9d and xe2x80x9cnon Uxe2x80x9d copolymers of alkyl acrylates and N-vinylpyrrolidone which comprise 5% to 50% of xe2x80x9cUxe2x80x9d monomers centred on C24 to C50, the characteristics and efficiency of the copolymers being defined by the choice of solvent/initiator pair. Preferred xe2x80x9cUxe2x80x9d co-monomers are alkyl acrylates centred on about C30.